Mild-hybrid vehicles: A near term technology trend for CO2 emissions reduction
This paper provides a comprehensive overview of mild-hybrid technology and recent developments in 48V mild hybridization of conventional vehicle powertrains. It estimates the CO2 reduction potential, as well as current and future system cost, for different mild-hybrid electric vehicle (MHEV) system configurations. It also reviews the market penetration trends and future projections of MHEV technology in Europe.
For mild-hybrid systems with comparable technical parameters, conservative estimates for the CO2 reduction potential under type-approval conditions compared with a baseline stop/start system range from about 7% for the P0 architecture to almost 16% for the more complex P2, P3, and P4+P0 architectures. Optimizing engine operation and increasing electric motor power can yield an additional 5% to 10% CO2 reduction on P2, P3, and P4 architectures.
The costs of mild-hybrid technologies increase with system complexity. The P0 manufacturing cost is conservatively projected to drop from €558 in 2020 to €338 in 2030. For systems with similar battery capacity and power, the cost for a mild-hybrid technology packet with P2 or P3 architecture is about 30% higher and a P4+P0 system added to a front-wheel drive vehicle is about 50% higher than for the P0 system.
The paper also derives the cost incurred for each percentage point CO2 emissions reduction. The increase in overall powertrain efficiency of more complex MHEV architectures, due to added functionality and advantageous positioning of the electric machine, outweighs the additional cost and results in lower cost per percentage point CO2 reduction compared to the P0 architecture. The P3 and P2 coaxial architectures are the most cost efficient, with over 40% lower cost than the P0 for the same CO2 reduction.
Besides improving fuel efficiency, mild hybridization also has the potential to reduce pollutant emissions. Brake particle emissions can be reduced when using the electric machine for regenerative braking. In addition, using the assistance of the electric machine during accelerations can reduce engine-out emissions. The higher power of a 48V system allows the application of electrically heated catalysts and electrical turbochargers, which can substantially reduce emissions at cold start and during transient operation.